Coupled inductors utilized for DC/DC converters, etc., have two coils wound around one core, and allow currents to flow through the two coils, respectively, so as to generate magnetic fluxes generated from the respective coils in opposite directions as disclosed in JP 2000-14136 A, JP 2002-291240 A, and JP 2010-62409 A.
According to such coupled inductors of this kind, multiple reactors can be integrated while suppressing an increase of the flux density. Hence, such coupled inductors can be downsized. Accordingly, such coupled inductors are widely applied as a switching power source for electronic devices like a personal computer.
In recent years, coupled inductors are sometimes employed in an application in which a large current is necessary, i.e., it is attempted that such inductors are applied as an inductor for vehicular devices that allow a current of several 10 to 100 A to flow therethrough. According to a large-current application, it is necessary that a saturated flux density of the core is high. When, however, the saturated flux density is low, the flux density is easily saturated within the applied range, and thus an inductance value decreases. The decrease of the inductance value results in an increase of a ripple current, increasing the reactor loss.
JP 2010-62409 A discloses the use of a ferrite core as the core of the coupled inductor. However, such a core is not suitable for a large-current application because of the following reasons.
One of the features of a ferrite core is that a saturated flux density is low in comparison with other metal magnetic materials. For example, pure iron: 2 T, sendust: 1.1 T, and Mn—Zn ferrite: 0.3 to 0.4 T. In addition, a ferrite core has a higher magnetic permeability than dust cores. That is, dust core: μ 50 to 200, and Mn—Zn ferrite core: equal to or greater than μ 1000. In order to cause a ferrite core with a low saturated flux density to cope with a large-current application, it is necessary to increase the cross-sectional area of the core, and to provide a large gap in order to decrease the effective magnetic permeability of the reactor.
When, however, the gap becomes large, leakage fluxes from the gap may interlink with a winding, an aluminum casing, etc., to generate an eddy current. This causes a loss. In addition, this may increase a possibility that an efficiency is decreased and heat is generated. The necessary of a large gap decreases an initial inductance value (at the time of OA), and thus a ripple current increases.
In the case of a dust core, the saturated flux density of the material itself is high, and the core itself has a low magnetic permeability. Accordingly, it is unnecessary to provide a large gap. Hence, the problem originating from the leakage flux and the reduction of the initial inductance value is avoidable. Accordingly, dust cores are excellent materials in comparison with ferrite cores, but a pure-iron-based dust core has a large core loss, and generates heat. Hence, dust cores are not suitable for a large-current application.
In a reactor characteristic, the maximum differential permeability represents an inductance (initial inductance value) when no load is applied (at the time of OA), but when this maximum differential permeability is too low, the initial inductance value becomes low, and thus a ripple current becomes large in a current waveform. When the ripple current becomes large, an effective current becomes also large, and thus the reactor loss becomes large, which may negatively affect other circuit components. According to conventional ferrite cores and dust cores, however, the maximum differential permeability is not usually taken into consideration, and it is difficult to overcome the aforementioned problems.
Several solutions to increase the initial inductance are possible, such as to increase the number of turns of winding, and to increase the cross-sectional area of the core, in addition to the maximum differential permeability, but those result in an increase in the size of the reactor. According to those countermeasures, a DC resistance increases, and thus a loss also increases. Accordingly, it is disadvantageous for reactors.
According to conventional coupled inductors, generation of heat is not a problem since a small current is caused to flow. Hence, coils formed of round magnet wires are popular. However, round magnet wires have a low winding space factor, and thus an inductor becomes large in size when applied to a large-current application. In addition, a coil is formed by turning the magnet wire in multiple layers, and thus the heat dissipation is not excellent.
It is an objective of the present disclosure to provide a coupled inductor that can satisfy both characteristics: saturated flux density; and reactor loss in a large-current application. It is another objective of the present disclosure to provide a coupled inductor that ensures an initial inductance value when no load is applied to be a predetermined value to reduce a ripple current, and that can decrease a loss.